Carbon dioxide is recognized as a greenhouse gas that contributes to climate changes. A major cause of CO2 emission is from combustion of fossil fuels like oil and coal in production of electricity and heat. The world consumption of coal is expected to increase by 49% in 2030 and accordingly much attention has been focused on reducing CO2 emission from power plant flue-gas streams. Currently, aqueous solutions of organic amines are being used to capture CO2 in scrubbers as carbamates despite concerns about, e.g. low absorbent capacity and energy intensive regeneration by desorption, which may require up to one-third of the total energy output from the power plant. Other technical challenges with amine sorbents include corrosion of steel pipes and pumps as well as thermal and chemical decomposition. In addition concern has been expressed about emission of the amines to the atmosphere leading to serious human health hazards. Solid absorbers also work unsatisfactorily among others due to the high desorption energy required to desorb CO2 and regenerate the absorber.
In order to overcome these inherent problems, it is highly desired to develop a viable and energy-efficient technology for CO2 capture by use of alternative, suitable absorbents.
Ionic liquids (ILs) are promising candidates as absorbents in CO2 removal due to their relatively high thermal stability, exceptionally low vapour pressure and tunable physicochemical properties [see eg. a) F. Jutz et al. Chem. Rev. 2011, 111, 322; b) J. Huang et al. Aust. J. Chem. 2009, 62, 298]. There have been many reports on CO2 capture using common ILs (typically referred to as first-generation ionic liquids), and a maximum CO2 absorption capacity of 0.75 in mole fraction of CO2 has been found for [C8MIM][PF6] at 40° C. under 93 bar pressure [see eg. a) L. A. Blanchard, Z. Gu, Joan F. Brennecke, J. Phys. Chem. B 2001, 105, 2437; b) Z. Gu, Joan F. Brennecke, J. Chem. Eng. Data 2002, 47, 339; c) Sudhir N. V. K. Aki, Berlyn R. Mellein, Eric M. Saurer, Joan F. Brennecke, J. Phys. Chem. B 2004, 108, 20355]. The CO2 absorption capacity of these kinds of ILs is, however, limited due to the relatively weak physisorption taking place between the IL and CO2. To circumvent this drawback, Davis and co-workers developed task-specific ionic liquids (TSILs) which are able to chemically bind CO2 to amine-functionalised imidazolium-based IL at ambient conditions. Even though these TSILs form a chemical bond with CO2 the absorption capacity was only 0.5 mol of CO2 per mol of IL (mole ratio of 0.33) due to intermolecular carbamate formation (1:2 mechanism), [see Eleanor D. Bates et al., J. Am. Chem. Soc. 2002, 124, 926]. Zhang et al. have proposed a new mechanism for the formation of carbamic acid after CO2 absorption in phosphonium-based amino acid functionalised ILs [J. Zhang, S. Zhang, K. Dong, Y. Zhang, Y. Shen, X. Lv. Chem. Eur. J. 2006, 12, 4021]. In this mechanism, one mol of CO2 also reacted with two moles of IL, however in presence of water (1 wt. %) the IL could absorb an equimolar amount of CO2 via a bicarbonate mechanism (mole ratio of 0.5). Recently, the formation of carbamic acid in amino acid-based ILs was further confirmed by Brennecke and co-workers by examining proline and methionine functionalised phosphonium-based ILs for CO2 absorption. Here it was shown that one mol of IL can absorb one mol of CO2 (1:1 mechanism) [see B. E. Gurkan, J. C. de la Fuente, E. M. Mindrup, L. E. Ficke, B. F. Goodrich, E. A. Price, W. F. Schneider, J. F. Brennecke, J. Am. Chem. Soc. 2010, 132, 2116]. A few other reports have also published equimolar amount of CO2 absorption in functionalised ILs, but the typical CO2-absorption capacity for published IL based solutions is sub-stoichiometric rather than super-stoichiometric.
Chemical absorption (chemisorption) of CO2 was first observed in imidazolium acetates, where the CO2 can be trapped either as bicarbonate or, as recently found, also as a carboxylate on carbon position 2 of the imidazole via a carbene mechanism [U.S. patent application Ser. No. 10/737,090; U.S. Pat. No. 5,336,298; Maginn, E., DOE Report, quarterly, Jan. 5-Mar. 5, 2005, 1-12, DOE Scientific and Technical Information, Oak Ridge, Tenn.]. This was recently proven by Rogers and co-workers upon determination of its crystal structure [Gurau G et al. Angew Chem Int Ed, 2011, 50:12024-12026]. However, amine functionalized ILs have proven to have a higher CO2 uptake stoichiometry and has attracted more attention. If an amine functionality is linked to the cation, intermolecular carbamate formation takes place resulting in a maximum stoichiometry of 0.5 moles of CO2 per mol of IL [aGalan Sanchez L et al., Solvent properties of functionalized ionic liquids for CO2 absorption. Chem Engineer Res Design, 2007, 85: 31-39; bSoutullo M et al. Reversible CO2 capture by unexpected plastic-, resin-, and gel-like ionic soft materials was discovered during the combi-click generation of a TSIL library. Chem Mater, 2007, 19: 3581-3583]. This has been demonstrated for imidazolium-based cations, where the amine is introduced on one of the alkyl chains. A major drawback of this method is that the already high viscosity of the IL becomes much higher when attaching another functional group to the IL. The important point is, however, that these compounds show much higher CO2 uptake than their non-functionalized counterparts.
If the amine functionality is attached to the anion of the IL, carbamate species also form, but there is a possibility that the negatively charged functionality on the anion can take up the proton released upon CO2 capture forming carbamic acid. Then CO2 can be absorbed with a stoichiometry of up to 1:1, meaning much more efficient use of the IL. However, in most cases, the absorption capacity still corresponds to 0.5:1. This could very well be related to residual water in the IL interfering with the released proton, preventing it from protonating the anionic part.
Therefore it must be concluded that despite growing demand, there has not yet been suggested a satisfactory and economically feasible technical solution to the problem of removing CO2 from eg. flue gases which has a high sorption efficiency, a high capacity and a sufficient robustness to allow for uninterrupted performance over long time periods.